1. Technical Field
This invention relates generally to exhaust gas recirculation systems for internal combustion engines, and more particularly to a hydraulically-actuated exhaust gas recirculation pump in conjunction with a hydraulically-assisted turbocharger for an engine.
2. Background Art
It is desirable to recirculate the exhaust gas of internal combustion engines, and particularly heavy-duty diesel engines, to reduce undesirable NO.sub.x emissions without increasing particulate material (PM) emissions. However, in turbocharged engines, there is an adverse pressure gradient between the exhaust and intake manifold so that some means is required to pump and control exhaust gas recirculation (EGR) flow. In particular, in heavy-duty diesel engines, EGR reduces engine air/fuel ratio (A/F) which increases particulate formation under some operating conditions. For example under peak torque conditions, it is desirable to increase the A/F when using EGR to improve the NO.sub.x -PM trade-off. Also, it is important that engine emissions be controlled during transient conditions. Transient conditions exist when an engine moves from one load state to another. For example, an "up transient" occurs when an engine moves from a low load (relatively high A/F) to a higher load (lower A/F) condition. Engine speed may also change during a transient. Additionally, there is generally a deterioration of transient performance in any engine having exhaust gas recirculation (EGR). Adding energy to the turbocharger during transients reduces smoke and particulate emissions.
Therefore, there are three main problems with using exhaust gas recirculation in general, and in turbocharged heavy-duty diesel engines in particular. First, a method must be provided to drive, or pump, the recirculated exhaust gas from the exhaust manifold to the intake manifold of the engine. Secondly, additional air should be provided under some EGR conditions, such as peak torque, to improve the NO.sub.x -PM trade-off. Thirdly, a method of overcoming the deterioration of transient performance of the engine must be provided. Additional air should be added during up transients to clear the EGR from the intake system and increase the A/F to reduce smoke and particulates.
Several arrangements have been proposed for providing an hydraulic assist to a conventional turbocharger for the purpose of improving transient performance of an engine. For example, U.S. Pat. No. 3,869,866 issued Mar. 11, 1975 to Seamus G. Timoney, describes an internal combustion engine having a hydraulically-assisted turbocharger. However, there has heretofore been no system provided which works in conjunction with an auxiliary-boosted turbocharger to pump a portion of the exhaust gas discharged from the turbocharger turbine exhaust port or exhaust manifold into the intake manifold of the engine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an exhaust gas recirculation system suitable for use in a turbocharged engine. It is also desirable to have such an exhaust gas recirculation system that, in conjunction with a hydraulically-assisted turbocharger, improves the transient performance of an engine, and the performance under EGR conditions where the A/F is low due to EGR.